Fine solid solution alloy particles and method for producing same

ABSTRACT

The alloy fine particles of the present invention are fine particles of a solid solution alloy, in which a plurality of metal elements are mixed at the atomic level. The production method of the present invention is a method for producing alloy fine particles composed of a plurality of metal elements. This production method includes the steps of (i) preparing a solution containing ions of the plurality of metal elements and a liquid containing a reducing agent; and (ii) mixing the solution with the liquid that has been heated.

TECHNICAL FIELD

The present invention relates to solid solution alloy fine particles anda method for producing the same.

BACKGROUND ART

Alloys exhibit different properties from those of individual constituentmetal elements. Therefore, newly developed alloys are expected to haveproperties (for example, catalytic properties) that conventional metalsdo not have. On the other hand, metal fine particles are expected tohave a variety of applications for reasons such as their large specificareas and possibly different properties and structures from those ofbulk metals. For these reasons, various alloy fine particles have beenstudied. For example, a method for producing alloy particles containingsilver and rhodium is disclosed (see Non Patent Literature 1).

CITATION LIST Non Patent Literature

Non Patent Literature 1

-   Paper No. 4L2-36, Proceedings (I) of the 88th Spring Meeting of The    Chemical Society of Japan, 2008

SUMMARY OF INVENTION Technical Problem

As shown in a phase diagram of FIG. 18, however, silver and rhodium inbulk form do not form a solid solution at the atomic level. Even if amixture of silver and rhodium is heated and melted, silver and rhodiumremain separated. Therefore, even if a melt containing silver andrhodium is cooled rapidly, it is difficult to produce an alloy in whichsilver and rhodium form a solid solution. On the other hand, in themethod of Non Patent Literature 1, silver ions and rhodium ions arereduced in a solution to produce fine particles. In this method of NonPatent Literature 1, however, it is difficult to produce fine particlesin which silver and rhodium form a solid solution at the atomic level.Nevertheless, if silver and rhodium do not form a solid solution at theatomic level, the resulting alloy is unlikely to exhibit its uniqueproperties. FIG. 19 shows a phase diagram of gold and rhodium. As isclear from the phase diagram of FIG. 19, it is difficult to produce asolid solution alloy of gold and rhodium.

Under these circumstances, it is one of the objects of the presentinvention to provide alloy fine particles in which a plurality of metalelements are mixed at the atomic level and a method for producing thesame.

Solution to Problem

In order to achieve the above object, the alloy fine particles of thepresent invention are fine particles of a solid solution alloy, in whicha plurality of metal elements are mixed at the atomic level. The phrase“mixed at the atomic level” means that, in one aspect, individualelements are randomly dispersed in an elemental map obtained using aSTEM with a spatial resolution of 0.105 nm, and in another aspect, asingle peak pattern is observed by XRD.

The production method of the present invention is a method for producingalloy fine particles composed of a plurality of metal elements. Thisproduction method includes the steps of (i) preparing a solutioncontaining ions of the plurality of metal elements and a liquidcontaining a reducing agent; and (ii) mixing the solution with theliquid that has been heated.

Advantageous Effects of Invention

According to the present invention, solid solution alloy fine particlesin which a plurality of metal elements are mixed at the atomic level areobtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a step of the production method of thepresent invention.

FIG. 2 shows another example of the step of the production method of thepresent invention.

FIG. 3 shows an example of transmission electron micrographs of alloyfine particles produced in Example 1.

FIG. 4 shows another example of the transmission electron micrographs ofthe alloy fine particle produced in Example 1.

FIG. 5 shows an EDX spectrum of the alloy fine particle produced inExample 1.

FIG. 6 shows an XRD pattern of the alloy fine particles produced inExample 1.

FIG. 7 shows transmission electron micrographs of alloy fine particlesproduced in Examples 2 to 4.

FIG. 8 shows XRD patterns of the alloy fine particles produced inExamples 2 to 4.

FIG. 9 is a graph showing the relationship between the silver contentsand the lattice constants of the alloy fine particles produced inExamples 2 to 4.

FIG. 10 shows absorption spectra of the alloy fine particles produced inExamples 2 to 4.

FIG. 11 shows an XRD pattern of fine particles produced in ComparativeExample 1.

FIG. 12 shows an XRD pattern of fine particles produced in ComparativeExample 2.

FIG. 13 shows an XRD pattern of fine particles produced in ComparativeExample 3.

FIG. 14 shows an XRD pattern of alloy fine particles produced in Example5.

FIG. 15 shows an EDX spectrum and an electron micrograph of the alloyfine particle produced in Example 5.

FIG. 16A and FIG. 16B show the results obtained by observing the alloyfine particles produced in Example 1 with a scanning transmissionelectron microscope (STEM).

FIG. 17A and FIG. 17B show the results obtained by observing the alloyfine particles produced in Example 5 with a scanning transmissionelectron microscope.

FIG. 18 is a phase diagram of silver and rhodium.

FIG. 19 is a phase diagram of gold and rhodium.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, embodiments of the present invention will be described byway of examples. The present invention is not limited to the followingembodiments and examples. In the following description, specificnumerical values or specific materials may be given by way of examples,but other numerical values or other materials may be used as long as theeffect of the present invention can be obtained.

[Production Method of Alloy Fine Particles]

The method of the present invention is a method for producing alloy fineparticles composed of a plurality of metal elements. According to thisproduction method, solid solution alloy fine particles in which aplurality of metal elements are mixed at the atomic level are obtained.The alloy fine particles obtained by this production method constituteone aspect of the alloy fine particles of the present invention.

The method of the present invention includes the following steps (i) and(ii). Hereinafter, the plurality of metal elements that constitute thealloy fine particles may sometimes be referred to as “a plurality ofmetal elements (E)”.

In the step (i), a solution containing ions of the plurality of metalelements (E) and a liquid containing a reducing agent are prepared.Hereinafter, the solution containing the plurality of metal elements (E)may sometimes be referred to as a “metal ion solution” or a “solution11”. The liquid containing a reducing agent may sometimes be referred toas a “liquid 12”.

The plurality of metal elements (E) may be two kinds of metal elements.In that case, binary alloy fine particles are obtained. When theplurality of metal elements (E) include rhodium, rhodium alloy fineparticles are obtained.

An example of the plurality of metal elements (E) is a combination ofsilver (Ag) and rhodium (Rh). Another example of the plurality of metalelements (E) is a combination of gold (Au) and rhodium (Rh).

The metal ion solution can be prepared by dissolving at least one typeof compound containing the plurality of metal elements (E) in a solvent.One compound may contain all the metal elements included in theplurality of metal elements (E). One compound also may contain only onemetal element included in the plurality of metal elements (E).

When the plurality of metal elements (E) are silver and rhodium, themetal ion solution can be prepared by dissolving a silver compound and arhodium compound in a solvent. Examples of the silver compound includesilver (I) acetate (AgCH₃COO) and silver nitrate (AgNO₃). Examples ofthe rhodium compound include rhodium (III) acetate (Rh(CH₃COO)₃) andrhodium (II) acetate (Rh(CH₃COO)₂). As the solvent, a solvent capable ofdissolving silver ions and rhodium ions is used. An example of thesolvent is water.

When the plurality of metal elements (E) are gold and rhodium, the metalion solution can be prepared by dissolving a gold compound and a rhodiumcompound in a solvent. Examples of the gold compound include chloroauricacid (HAuCl₄). Examples of the rhodium compound include theabove-mentioned rhodium compounds and rhodium (III) chloride (RhCl₃). Anexample of the solvent is water.

The concentration of ions of one of the plurality of metal elements (forexample, silver ions or gold ions) in the metal ion solution may be inthe range of 0.1 mmol/L to 1 mol/L (for example, in the range of 0.1mmol/L to 5 mmol/L). The concentration of rhodium ions in the metal ionsolution may be in the range of 0.1 mmol/L to 1 mol/L (for example, inthe range of 0.1 mmol/L to 5 mmol/L or in the range of 0.1 mmol/L to 1mmol/L).

The alloy composition can be varied by varying the ratio between theconcentration of silver ions C_(Ag) (mol/L) in the metal ion solutionand the concentration of rhodium ions C_(Rh) (mol/L) in the metal ionsolution. The value of C_(Rh)/[C_(Rh)+C_(Ag)] may be 0.1 or more, 0.2 ormore, 0.3 or more, 0.4 or more, 0.5 or more, 0.6 or more, 0.7 or more,0.8 or more, or 0.9 or more. The value may also be 0.9 or less, 0.8 orless, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less,0.2 or less, or 0.1 or less. The use of a metal ion solution withC_(Rh)/[C_(Rh)+C_(Ag)]=X makes it possible to produce alloy fineparticles having a rhodium content of almost 100× atomic %. For example,with the use of a metal ion solution with a C_(Rh)/[C_(Rh)+C_(Ag)] valueof 0.5 or more, alloy fine particles having a rhodium content of 50atomic % or more can be produced. When the plurality of metal elements(E) are two kinds of elements, the relationship between theconcentrations of ions of the individual elements in the metal ionsolution and the resulting alloy composition is the same as theabove-mentioned relationship between the concentrations of silver ionsand rhodium ions and the resulting alloy composition.

Next, in the step (ii), the metal ion solution (solution 11) is mixedwith the heated liquid (liquid 12) containing a reducing agent. In thestep (ii), not only the liquid 12 but also the solution 11 may beheated.

In the step (ii), the solution 11 may be mixed with the liquid 12 bydropping the solution 11 into the heated liquid 12. In the step (ii),the solution 11 may be mixed with the liquid 12 by spraying the solution11 onto the heated liquid 12. For example, in the step (ii), as shown inFIG. 1, the solution 11 and the heated liquid 12 may be mixed bydropping the former into the latter. In the step (ii), as shown in FIG.2, the solution 11 and the heated liquid 12 may be mixed by spraying theformer onto the latter. When the concentration of metal ions in themetal ion solution is high, it may be preferable in some cases to mixthe solution 11 and the liquid 12 by spraying the solution 11.

In the step (ii), the solution 11 may be mixed with the liquid 12 thathas been heated to a temperature not lower than a temperature at whicheach of the ions of the plurality of metal elements (E) is reduced.Furthermore, in the step (ii), the solution 11 may be mixed with theliquid 12 that has been heated to a temperature higher by 20° C. or morethan a temperature at which each of the ions of the plurality of metalelements (E) is reduced. In these two cases, the reducing agent may beethylene glycol.

Spraying of the solution 11 and/or the liquid 12 can be performed, forexample, using a spray gun or an ink jet head. The particle size of thealloy fine particles to be formed may possibly be controlled by varyingthe size of sprayed droplets.

The reducing agent contained in the liquid 12 may be alcohol. The liquid12 as a solvent may consist of an alcohol acting as a reducing agent(for example, ethylene glycol). The liquid 12 may contain an alcohol notacting as a reducing agent in addition to an alcohol acting as areducing agent. When the liquid 12 is heated, the action of the alcoholas a reducing agent is increased. The temperature to which the liquid 12is heated in the step (ii) depends on the type of alcohol as a reducingagent contained in the liquid 12. For example, when ethylene glycol isused, it is assumed that silver ions are reduced at 100° C. or lower andrhodium ions are reduced at around 140° C. Therefore, when the pluralityof metal elements (E) are silver and rhodium and ethylene glycol is usedas a reducing agent, the liquid 12 must be heated to 140° C. or higher.

There is no limitation on the type of alcohol as a reducing agentcontained in the liquid 12 as long as the effect of the presentinvention can be obtained. The alcohol used as a reducing agent may be amonovalent alcohol, or a polyvalent alcohol such as a divalent alcohol.Preferred examples of the alcohol used as a reducing agent are at leastone type of alcohol selected from the group consisting of ethyleneglycol, diethylene glycol, and triethylene glycol. Hereinafter, at leastone type of alcohol selected from the group consisting of ethyleneglycol, diethylene glycol, and triethylene glycol may sometimes bereferred to as “ethylene glycols”. The boiling point of ethylene glycolsis 190° C. or higher. Therefore, the use of such an alcohol as a solventmakes it possible to produce alloy fine particles at a high temperature.

Instead of the alcohol (i.e. an alcohol acting as a reducing agent)contained in the liquid 12, a substance capable of reducing metal ions(such as silver ions, rhodium ions, and gold ions) and acting as asolvent may be used.

At least one selected from the solution 11 and the liquid 12 may containa protective agent for preventing the agglomeration of the alloy fineparticles. The use of a protective agent makes it easier to obtain alloyfine particles of small size. Specifically, both or either one of thesolution 11 and the liquid 12 may contain a protective agent. Examplesof the protective agent include polymers and surfactants. For example,the protective agent is poly(N-vinyl-2-pyrrolidone) (hereinafter may bereferred to as “polyvinylpyrrolidone” or “PVP”). The concentration ofthe protective agent in the solution is selected according to the typeof the protective agent. When the protective agent ispolyvinylpyrrolidone, it may be added so that the concentration of itsconstituent units (monomer units) is in the range of 0.1 mmol/L to 2mol/L (for example, in the range of 1 mmol/L to 10 mmol/L).

When neither the solution 11 nor the liquid 12 contains a protectiveagent, the alloy fine particles are likely to agglomerate to formparticles of larger size.

A typical example of the liquid 12 is a solution (an alcohol solution ofa protective agent) obtained by dissolving the protective agent in analcohol (for example, ethylene glycols). For example, an ethylene glycolsolution in which polyvinylpyrrolidone is dissolved can be used as theliquid 12. Hereinafter, the liquid 12 in which the protective agent isdissolved may sometimes be referred to as a “reducing agent solution”.

In one example, the reducing agent is ethylene glycol, the plurality ofmetal elements (E) are silver and rhodium, and in the step (ii), thesolution 11 is mixed with the liquid 12 that has been heated to 145° C.or higher. In another example, the reducing agent is ethylene glycol,the plurality of metal elements (E) are gold and rhodium, and in thestep (ii), the solution 11 is mixed with the liquid 12 that has beenheated to 145° C. or higher. In these examples, the liquid 12 may be anethylene glycol solution in which polyvinylpyrrolidone is dissolved. Thesolution 11 may be an aqueous solution containing silver ions andrhodium ions or an aqueous solution containing gold ions and rhodiumions.

In the production method of the present invention, the liquid 12 may beone essentially or substantially free from a reducing agent (forexample, sodium borohydride (NaBH₄) or hydrazine) other than alcohol.However, sodium borohydride or the like may be used as a reducing agentas long as the effect of the present invention can be obtained.

When the alcohol contained in the liquid 12 is ethylene glycol, theliquid 12 may be heated to a temperature of 145° C. or higher. It mayalso be heated to a temperature of 150° C. or higher or 160° C. orhigher. In the step (ii), the liquid 12 may be heated to a lowertemperature as long as the effect of the present invention can beobtained. In the step (ii), the liquid 12 may be heated to a temperatureof 200° C. or lower, for example, 50° C. or lower.

In the step (ii), the solution 11 and the liquid 12 are mixed in such away that the temperature of the liquid 12 does not drop excessively. Forexample, when the alcohol is ethylene glycol, the solution 11 and theliquid 12 are mixed in such a way that the temperature of the liquid 12is maintained at 145° C. or higher, 150° C. or higher, or 160° C. orhigher. A way of preventing an excessive drop in the temperature of theliquid 12 is, for example, to add the solution 11 little by little.Examples of methods of adding the solution 11 little by little include amethod of dropping the solution 11 and a method of spraying the solution11. The solution 11 may also be added after it is heated to a certaintemperature.

In one example, the weight of the solution 11 to be added per second tothe liquid 12 may be not more than one three-hundredth (for example, notmore than one three-thousandth) of the weight of the liquid 12.

According to the production method of the present invention, solidsolution alloy fine particles in which the plurality of metal elements(E) are mixed at the atomic level are obtained. For example, fineparticles of a silver-rhodium solid solution alloy, in which silver andrhodium are mixed at the atomic level, are obtained. Silver and rhodiumin bulk form do not form a solid solution at the atomic level. However,fine particles having a particle size of several tens of nanometers orless have different structures and properties from those of bulk metals,and it is believed that silver and rhodium therein can form a solidsolution at the atomic level. Furthermore, according to the presentinvention, fine particles of a gold-rhodium solid solution alloy, inwhich gold and rhodium are mixed at the atomic level, are obtained.

Even if the plurality of metal elements (E) are a plurality of metalelements that do not form a solid solution even in the liquid phase in aphase diagram, the production method of the present invention makes itpossible to obtain alloy fine particles in which the plurality of metalelements (E) form a solid solution at the atomic level. In this case,the metal ion solution may be a solution containing a plurality of metalelements whose concentrations correspond to the composition ratio ofthose metals in bulk form that do not form a solid solution. Thisproduction method makes it possible to obtain alloy fine particles inwhich a plurality of metal elements form a solid solution at the atomiclevel although these metal elements have a composition ratio that doesnot allow them to form a solid solution in the liquid phase if they arein bulk form (i.e., these metal elements include a plurality of metalelements that do not form a solid solution in the liquid phase over theentire range of composition ratios when they are in bulk form). Theproduction method of the present invention can be used for producingvarious alloy fine particles.

[Alloy Fine Particles]

The alloy fine particles of the present invention are alloy fineparticles in which the plurality of metal elements (E) form a solidsolution. More specifically, the alloy fine particles of the presentinvention are solid solution alloy fine particles in which the pluralityof metal elements (E) are mixed at the atomic level. The fact that theyare solid solution alloy fine particles in which the plurality of metalelements (E) are mixed at the atomic level can be confirmed bymeasurements or the like performed in the following examples. Examplesof the alloy fine particles of the present invention include rhodiumalloy fine particles containing rhodium. Examples of the alloy fineparticles of the present invention include silver-rhodium alloy fineparticles and gold-rhodium alloy fine particles.

The alloy fine particles of the present invention can be produced by theproduction method of the present invention. Since the details of theproduction method of the present invention that have been described canbe applied to the alloy fine particles of the present invention,overlapping descriptions may be omitted. Furthermore, the details of thealloy fine particles of the present invention that have been describedcan be applied to the production method of the present invention.

In one aspect, the alloy fine particles of the present invention aresuch that elemental mapping using a scanning transmission electronmicroscope with a resolution of 0.105 nm demonstrates that there is nophase separation in the alloy fine particles.

The alloy fine particles of the present invention (for example, binaryalloy fine particles) may be such that all of the plurality of metalelements (E) are contained in any cube with a side length of 1 nm thatis arbitrarily selected from the alloy fine particle.

In one aspect, the alloy fine particles of the present invention aresuch that X-ray diffraction demonstrates that there is no phaseseparation in the alloy fine particles.

In the rhodium alloy fine particles of the present invention (forexample, silver-rhodium alloy fine particles and gold-rhodium alloy fineparticles), the rhodium content may be 10 atomic % or more, 20 atomic %or more, 30 atomic % or more, 40 atomic % or more, 50 atomic % or more,60 atomic % or more, 70 atomic % or more, 80 atomic % or more, or 90atomic % or more. It may also be 90 atomic % or less, 80 atomic % orless, 70 atomic % or less, 60 atomic % or less, 50 atomic % or less, 40atomic % or less, 30 atomic % or less, 20 atomic % or less, or 10 atomic% or less.

There is no limitation on the particle size of the alloy fine particlesof the present invention as long as the plurality of metal elements (E)form a solid solution at the atomic level. The alloy fine particles ofthe present invention (for example, silver-rhodium alloy fine particlesand gold-rhodium alloy fine particles) may have an average particle sizeof 30 nm or less, 20 nm or less, or 10 nm or less. The average particlesize may be 3 nm or more. The average particle size can be calculated ina manner described in the examples.

The alloy fine particles of the present invention may be composed of theplurality of metal elements (E) that do not form a solid solution evenin the liquid phase.

The alloy fine particles of the present invention may contain traceamounts of impurities as long as they do not essentially change theproperties of the particles.

EXAMPLES

Hereinafter, the present invention will be described in more detail byway of examples. In the examples and comparative examples below, anelectron microscope (JEM 2010EFE manufactured by JEOL Ltd.) and ascanning transmission electron microscope (HD-2700 manufactured byHitachi High-Technologies Corporation) were used for EDX measurements.An X-ray diffractometer (D8 ADVANCE manufactured by Bruker AXS) andSPring-8 BL02B2 were used for XRD measurements. As a scanningtransmission electron microscope, HD-2700 with a resolution of 0.105 nm,manufactured by Hitachi High-Technologies Corporation, was used.Elemental mapping was conducted with EDX. In the following examples,elemental mapping data were obtained using a scanning transmissionelectron microscope (HD-2700). In the elemental mapping performed in thefollowing examples, an electron beam was scanned in two dimensions usingthe STEM to generate a scan image while the EDX incorporated in the STEMdetected the elements, which were plotted in two dimensions withreference to the operation of the STEM to conduct the elemental mapping.

Example 1

In Example 1, silver-rhodium alloy fine particles were produced bydropping the solution 11.

First, polyvinylpyrrolidone (0.1 mmol) was dissolved in ethylene glycol(100 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.005 mmol) and rhodium (III) acetate (0.005 mmol) were dissolved in 20ml of pure water to obtain a metal ion solution (solution 11).

Next, the reducing agent solution was heated, and when the temperatureof the reducing agent solution reached 160° C., the metal ion solutionwas dropped with a syringe into the reducing agent solution. At thistime, the reducing agent solution was dropped in such a way that thetemperature of the reducing agent solution was maintained at 160° C. orhigher. Next, the reducing agent solution into which the metal ionsolution was dropped was centrifuged to separate the reaction product(fine particles).

FIG. 3 shows the transmission electron micrograph and the particle sizedistribution of the fine particles of Example 1. As shown in FIG. 3, thefine particles having a uniform particle size were obtained. The averageparticle size of the fine particles of Example 1 was 12.5 nm±2.6 nm. Theaverage particle size was calculated by actually measuring the particlesizes of (at least 300) particles in the transmission electronmicroscope photograph (TEM photograph) and averaging them. FIG. 4 showsthe transmission electron microscope photograph of one of the fineparticles of Example 1. Since regularly-spaced lattice fringes areobserved across the fine particle, the fine particle in FIG. 4 isconsidered as a single crystal.

FIG. 5 shows the spectrum of the fine particle shown in FIG. 4, obtainedby energy-dispersive X-ray spectroscopy (EDX). The result shown in FIG.5 indicates that silver and rhodium are present in one particle in aratio of approximately 1:1, which demonstrates that the fine particlesof Example 1 are alloy fine particles in which silver and rhodium form asolid solution at the atomic level.

FIG. 6 shows the XRD pattern (X-ray diffraction pattern) of the fineparticles of Example 1. The fitting curve shown in FIG. 6 is a curveobtained by assuming that the alloy fine particles of Example 1 have anfcc structure. This fitting curve coincides approximately with that ofmeasured values, which indicates that the alloy fine particles ofExample 1 have an fcc structure. Furthermore, each peak of the fineparticles of Example 1 appears between the peak of bulk silver and thepeak of bulk rhodium. This result also indicates that the fine particlesof Example 1 are alloy fine particles in which silver and rhodium form asolid solution at the atomic level.

Example 2

In Example 2, alloy fine particles containing silver and rhodium in anatomic ratio of approximately 50:50 were produced by spraying thesolution 11.

First, polyvinylpyrrolidone (1.0 mmol) was dissolved in ethylene glycol(200 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.05 mmol) and rhodium (III) acetate (0.05 mmol) were dissolved in 20ml of pure water to obtain a metal ion solution (solution 11).

Next, the reducing agent solution was heated, and when the temperatureof the reducing agent solution reached 160° C., the metal ion solutionwas sprayed with a spray gun onto the reducing agent solution. At thistime, the reducing agent solution was sprayed in such a way that thetemperature of the reducing agent solution was maintained at 160° C. orhigher. Next, the reducing agent solution into which the metal ionsolution was added was centrifuged to separate the reaction product(fine particles).

Example 3

In Example 3, alloy fine particles containing silver and rhodium in anatomic ratio of approximately 75:25 were produced by spraying thesolution 11.

First, polyvinylpyrrolidone (1.0 mmol) was dissolved in ethylene glycol(200 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.075 mmol) and rhodium (III) acetate (0.025 mmol) were dissolved in 20ml of pure water to obtain a metal ion solution (solution 11).

Next, the reducing agent solution was heated, and when the temperatureof the reducing agent solution reached 160° C., the metal ion solutionwas sprayed with a spray gun onto the reducing agent solution. At thistime, the reducing agent solution was sprayed in such a way that thetemperature of the reducing agent solution was maintained at 160° C. orhigher. Next, the reducing agent solution into which the metal ionsolution was added was centrifuged to separate the reaction product(fine particles).

Example 4

In Example 4, alloy fine particles containing silver and rhodium in anatomic ratio of approximately 25:75 were produced by spraying thesolution 11.

First, polyvinylpyrrolidone (1.0 mmol) was dissolved in ethylene glycol(200 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.025 mmol) and rhodium (III) acetate (0.075 mmol) were dissolved in 20ml of pure water to obtain a metal ion solution (solution 11).

Next, the reducing agent solution was heated, and when the temperatureof the reducing agent solution reached 160° C., the metal ion solutionwas sprayed with a spray gun onto the reducing agent solution. At thistime, the reducing agent solution was sprayed in such a way that thetemperature of the reducing agent solution was maintained at 160° C. orhigher. Next, the reducing agent solution into which the metal ionsolution was added was centrifuged to separate the reaction product(fine particles).

FIG. 7 shows the transmission electron micrographs of the fine particlesof Examples 2 to 4. As shown in FIG. 7, the particle size increases asthe proportion of silver increases under the same conditions.

FIG. 8 shows the XRD patterns of the fine particles of Examples 2 to 4.FIG. 8 also shows the results of silver fine particles and rhodium fineparticles. The results shown in FIG. 8 indicate that the fine particlesof Examples 2 to 4 are solid solution alloy fine particles and that allof the fine particles of Examples 2 to 4 have an fcc structure. FIG. 9shows the lattice constants estimated from the results of the X-raydiffraction measurements. As shown in FIG. 9, the lattice constantincreases continuously as the silver content increases.

FIG. 10 shows the measurement results of the absorption spectra of thefine particles of Examples 2 to 4. FIG. 10 also shows the absorptionspectra of silver fine particles and rhodium fine particles. In theabsorption spectrum of silver fine particles, an absorption peak due tothe surface plasma absorption appears around 400 nm. On the other hand,in the cases of the fine particles of Examples 2 to 4, the absorptionpeak shifts to the shorter wavelengths and becomes broader as therhodium content increases. This result also suggests that silver-rhodiumalloy fine particles in which silver and rhodium form a solid solutionat the atomic level were obtained.

The above results demonstrate that the fine particles of Examples 1 to 4are solid solution alloy fine particles in which silver and rhodium aremixed at the atomic level.

Comparative Example 1

In Comparative Example 1, fine particles were produced by adding thesolution 11 to the liquid 12 in advance and then the resulting mixedsolution was heated from about room temperature (about 20° C.) to 140°C.

First, polyvinylpyrrolidone (10 mmol) was dissolved in ethylene glycol(100 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.5 mmol) and rhodium (III) acetate (0.5 mmol) were dissolved in 10 mlof pure water to obtain a metal ion solution (solution 11).

Next, the metal ion solution was added to the reducing agent solution,and then the resulting mixed solution was heated to 140° C. withstirring. Then, the mixed solution was stirred for one hour with itstemperature maintained at 140° C. Next, after the reaction, the mixedsolution was centrifuged to separate the reaction product (fineparticles).

FIG. 11 shows the XRD pattern of the fine particles of ComparativeExample 1. FIG. 11 also shows a curve fitted using fitting components 1and 2. The fitting component 1 is a component having a lattice constantof 4.08 angstroms and a particle size of 9.7 nm. The fitting component 2is a component having a lattice constant of 3.73 angstroms and aparticle size of 1.1 nm. The lattice constant of the fitting component 1is close to that of bulk silver (i.e., 4.086 angstroms) and the latticeconstant of the fitting component 2 is close to that of bulk rhodium(i.e., 3.803 angstroms). From the result shown in FIG. 11, it isbelieved that the fine particles of Comparative Example 1 are core-shellfine particles having a silver core or fine particles in which silverand rhodium are phase-separated.

Comparative Example 2

First, polyvinylpyrrolidone (0.15 mmol) was dissolved in ethylene glycol(100 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.03 mmol) and rhodium (III) acetate (0.03 mmol) were dissolved in 20ml of pure water to obtain a metal ion solution (solution 11).

Next, the reducing agent solution was heated, and when the temperatureof the reducing agent solution reached 140° C., the metal ion solutionwas dropped with a syringe into the reducing agent solution. At thistime, the reducing agent solution was dropped in such a way that thetemperature of the reducing agent solution was maintained at 140° C.Next, the reducing agent solution into which the metal ion solution wasdropped was centrifuged to separate the reaction product (fineparticles).

FIG. 12 shows the XRD pattern of the fine particles of ComparativeExample 2. FIG. 12 also shows a curve fitted using fitting components 1and 2. The fitting component 1 is a component having a lattice constantof 4.04 angstroms and a particle size of 5.3 nm. The fitting component 2is a component having a lattice constant of 3.89 angstroms and aparticle size of 3.5 nm. In the XRD pattern of the fine particles ofComparative Example 2, the peak is not that of a single fcc structurebut is composed of two components. The lattice constant of the fittingcomponent 1 is close to that of silver, and the lattice constant of thefitting component 2 is close to that of bulk rhodium. Therefore, it isbelieved that silver and rhodium are phase-separated in the fineparticles of Comparative Example 2.

Comparative Example 3

First, polyvinylpyrrolidone (0.1 mmol) was dissolved in ethylene glycol(100 ml) to obtain a reducing agent solution (liquid 12). Silver acetate(0.005 mmol) and rhodium (III) acetate (0.005 mmol) were dissolved in 20ml of pure water to obtain a metal ion solution (solution 11).

Next, the metal ion solution at room temperature was dropped with asyringe into the reducing agent solution at room temperature (about 20°C.). Next, the reducing agent solution into which the metal ion solutionwas dropped was heated to reflux with stirring at 160° C. for one hour.Next, the heated solution was centrifuged to separate the reactionproduct (fine particles). FIG. 13 shows the X-ray diffraction pattern ofthe fine particles thus obtained. In FIG. 13, the fitting component 1 isa component having a lattice constant of 4.070 angstroms and a particlesize of 5.4 nm. The fitting component 2 is a component having a latticeconstant of 3.842 angstroms and a particle size of 1.6 nm. From theX-ray diffraction pattern of FIG. 13, it is believed that silver andrhodium are phase-separated in the fine particles of Comparative Example3.

Example 5

In Example 5, gold-rhodium alloy fine particles were produced bydropping the solution 11.

First, polyvinylpyrrolidone (1.0 mmol) was dissolved in ethylene glycol(200 ml) to obtain a reducing agent solution (liquid 12). Chloroauricacid (0.05 mmol) and rhodium (III) chloride (0.05 mmol) were dissolvedin 20 ml of pure water to obtain a metal ion solution (solution 11).

Next, the reducing agent solution was heated, and when the temperatureof the reducing agent solution reached 160° C., the metal ion solutionwas sprayed with a spray gun onto the reducing agent solution. At thistime, the reducing agent solution was sprayed in such a way that thetemperature of the reducing agent solution was maintained at 160° C. orhigher. Next, the reducing agent solution onto which the metal ionsolution was sprayed was centrifuged to separate the reaction product(fine particles of Example 5).

FIG. 14 shows the X-ray diffraction pattern of the fine particles ofExample 5. FIG. 15 shows the EDX spectrum of the fine particles ofExample 5. FIG. 15 also shows an electron micrograph of a measured fineparticle. Not only the XRD pattern of Example 5 indicates a single fccpattern but also its lattice constant has a value between the latticeconstant of gold nanoparticles and that of rhodium nanoparticles. Thesefacts prove that gold and rhodium form a solid solution at the atomiclevel. Furthermore, the EDX spectrum confirms that both of the elements,i.e., gold and rhodium, are present in one particle.

[Observation with STEM]

The silver-rhodium alloy fine particles of Example 1 were observed usinga scanning transmission electron microscope (STEM). FIG. 16A and FIG.16B show the data of the fine particles of Example 1. In FIG. 16A, a)shows a dark-field STEM image, and b) to d) show elemental mapping data.FIG. 16B shows the result of line analysis. A scale bar in each of theimages in FIG. 16A indicates 10 nm. FIG. 16A shows that all of theparticles form a solid solution. Furthermore, FIG. 16B shows that theindividual elements are not locally present in a particle but both ofthe elements are uniformly distributed across the particle. In otherwords, the data of FIG. 16A and FIG. 16B indicate that silver andrhodium form a solid solution at the atomic level in the fine particlesof Example 1.

The fine particles of Example 5 were observed using a STEM. FIG. 17A andFIG. 17B show the data thus obtained. In FIG. 17A, a) shows a dark-fieldSTEM image, and b) to d) show elemental mapping data. FIG. 17B shows theresult of line analysis. A scale bar in each of the images in FIG. 17Aindicates 10 nm. FIG. 17A shows that all of the particles form a solidsolution. Furthermore, FIG. 17B shows that the individual elements arenot locally present in a particle but both of the elements are uniformlydistributed across the particle. In other words, the data of FIG. 17Aand FIG. 17B indicate that gold and rhodium form a solid solution at theatomic level in the fine particles of Example 5.

As shown in the above examples, according to the production method ofthe present invention, silver-rhodium fine particles in which silver andrhodium form a solid solution and gold-rhodium fine particles in whichgold and rhodium form a solid solution were obtained. No data have beenpresented to indicate that these elements are mixed at the atomic level.The present inventors have presented the first data indicating thatthese elements are mixed at the atomic level.

INDUSTRIAL APPLICABILITY

According to the present invention, solid solution alloy fine particlesin which a plurality of metal elements are mixed at the atomic level areobtained. These alloy fine particles can be used for variousapplications (for example, catalysts). For example, silver-rhodium alloyfine particles can be used as a catalyst for organic synthesis, anelectrode catalyst for a fuel cell, and a catalyst for reducing NO_(x).Furthermore, since silver-rhodium alloy fine particles are considered toexhibit hydrogen storage properties, they are expected to be applied tovarious devices by taking advantage of their hydrogen storageproperties. Silver-rhodium alloy fine particles in which silver andrhodium form a solid solution at the atomic level are expected toexhibit the properties similar to those of palladium. Likewise, it ispossible to produce alloys having various properties by producing alloysof various elements.

1-17. (canceled)
 18. Fine particles of a solid solution alloy, in whicha plurality of metal elements are mixed at the atomic level wherein theplurality of metal elements are a plurality of metal elements that donot form a solid solution even in the liquid phase.
 19. The alloy fineparticles according to claim 18, wherein the plurality of metal elementsare two kinds of metal elements.
 20. The alloy fine particles accordingto claim 19, wherein elemental mapping using a scanning transmissionelectron microscope with a resolution of 0.105 nm demonstrates thatthere is no phase separation in the alloy fine particles.
 21. The alloyfine particles according to claim 19, wherein X-ray diffractiondemonstrates that there is no phase separation in the alloy fineparticles.
 22. The alloy fine particles according to claim 20, whereinthe plurality of metal elements are silver and rhodium.
 23. The alloyfine particles according to claim 20, wherein the plurality of metalelements are gold and rhodium.
 24. The alloy fine particles according toclaim 18, having an average particle size of 20 nm or less.